The present invention relates to steam turbine buckets and particularly to a high strength steam turbine closure bucket for avoiding creep elongation.
Steam turbine buckets are currently designed using an identical material for all of the buckets for a particular stage of the turbine. The buckets are commonly formed of a Type 403 or 422 stainless steel, depending upon the stage temperature. Steam turbine buckets are also often designed for installation on the turbine wheel in a tangential direction. Consequently, the base or root of the steam bucket typically has a female dovetail configuration. The peripheral margin of the turbine wheel has a generally complementary male dovetail configuration and an insertion gap to facilitate installation of the buckets. For example, each bucket is assembled onto the rotor wheel by first locating the bucket base in the gap. Once the dovetails on the bucket and wheel are aligned, the bucket is displaced along the periphery of the wheel in a tangential direction. Additional buckets are similarly installed on the wheel and the roots or bases of the buckets stack against one another on the wheel margin. The installation process proceeds until the entire margin of the wheel, with the exception of the gap, is filled with buckets. The last bucket to be assembled to the wheel is called the closure bucket. The closure bucket typically does not have a dovetail. The closure bucket root or base may, however, be recessed to balance its weight with the weights of the other buckets about the wheel to balance the wheel. Because there is a gap in the dovetail margin of the wheel, the closure bucket must be secured to the adjacent buckets, i.e., the two buckets which straddle the gap and the closure bucket, in order to secure the closure bucket to the wheel. Typically, the closure bucket is attached to the adjacent buckets by pins extending in an axial direction engaging through the root or base portions of the adjacent buckets and the closure bucket.
It will be appreciated that during operation, the centrifugal load of the closure bucket is carried by the adjacent buckets through the pins. The applied loads on the closure and adjacent buckets are thus not uniform. High localized stresses are encountered at the location of the securement between the closure bucket and the two adjacent wheels, i.e., along the slots receiving the pins and the pins themselves. Consequently, creep and permanent deformation of the closure bucket and/or the adjacent buckets may occur after a period of operation at high temperatures and high centrifugal loads. For example, such high temperatures and loadings may occur in the reheat section of an intermediate stage turbine. As a result, the closure bucket may tend to elongate at its base or root in response to these high temperatures and stresses over time, with the result that the slot or hole for receiving the pins may elongate in a radial outward direction. Consequently, there is a need for an increase in the load-carrying capacity of at least the closure bucket in a steam turbine.
In accordance with a preferred embodiment of the present invention, the creep rupture strength of the closure bucket is increased in relation to the creep rupture strength of the majority of the remaining buckets disposed on the wheel. To increase the load-carrying capability of the closure bucket, the material of the closure bucket has a higher creep rupture strength than the majority of the buckets on the wheel. The closure bucket, for example, may be formed of a nickel-based alloy, while the majority of the remaining buckets are formed of a stainless steel. Further, because of the possibility of high local stresses at the juncture of the closure bucket and adjacent buckets, and in the pins, the pins and the two buckets straddling the closure bucket are likewise formed of a material having a higher creep rupture strength than the creep rupture strength of the remaining buckets about the wheel. Thus, a plurality of buckets about the wheel are formed of stainless steel and the closure bucket and preferably the two adjoining buckets and pins are formed of a higher strength material such as nickel-based alloy to avoid creep rupture.
In a preferred embodiment according to the present invention, there is provided a steam turbine wheel for use in a steam turbine, the wheel having a plurality of tangential entry buckets having tangentially extending dovetails for attachment to the wheel, the wheel having a generally complementary-shaped dovetail about a peripheral margin thereof and a gap in the margin for receiving the plurality of buckets and a closure bucket, the closure bucket being formed at least in part of a material having a creep rupture strength greater than the creep rupture strength of at least a majority of the plurality of buckets.
In a further preferred embodiment according to the present invention, there is provided a method of avoiding creep rupture strength failure of a first closure bucket in a tangential bucket entry turbine wheel having a peripheral dovetail-shaped margin and a plurality of tangential entry buckets having complementary dovetail-shaped bases secured to the wheel margin wherein the plurality of buckets and the first closure bucket are formed of the same materials, comprising the steps of removing the first closure bucket from the turbine wheel and securing a second closure bucket formed at least in part of a material having a higher strength than the material of the removed closure bucket to the turbine wheel in place of the first removed closure bucket.